This invention relates to a master cylinder having mechanical actuation for effecting a first brake application in response to an operator input and electronic actuation for effecting a second brake application in response to an input from an electronic control unit.
U.S. Pat. Nos. 5,018,353 and 5,111,661 disclose master cylinders wherein compensation between a first chamber in a bore of a housing and a reservoir occurs through a single center port compensation valve associate with a first piston and between a second chamber in the bore through a relationship established between a radial port and sealing structure carried on a second piston. In order to eliminate the possibility of cutting a nub in a seal by extrusion into the radial port, U.S. Pat. Nos. 5,207,062 and 5,279,125 disclose the use of a second center port compensation valve through which compensation is achieved for the second chamber in a bore. In master cylinders having such separate compensation valves for the first and second pressurizing chambers in the bore communication typically occurs through ports from separate passages in the housing when the first and second pistons are in a rest position. As the first and second pistons approach the rest position, stop pins engage the compensation valves to open the compensation valve and initiate communication between the reservoir and bore. Unfortunately, the passage required by the rear piston is quite long as the compensation port needs to be located adjacent the end of the housing for the master cylinder. In addition, when such master cylinders are recessed into a front chamber of a vacuum brake booster care needs to be taken with respect to sealing structure to assure that vacuum does not draw fluid into the vacuum brake booster.
U.S. Pat. No. 5,943,863 discloses a master cylinder having first and second center port compensation valves respectively associated with first and second caging arrangements that locates the first and second pistons in a bore of a master cylinder. In this master cylinder, each of the center port compensation valves has a poppet that extends from a stem that joins a first retainer to a second retainer to cage a return spring. In response to an input force applied to a first piston by an operator, the first and second pistons move in the bore to initially compress the return springs and thereafter allow the poppet to engage a seat and seal first and second chambers to thereafter develop pressurized fluid and effect a brake application. This type master cylinder functions in an adequate manner for service brake application, however, when additional functions are introduced into a brake system, such as traction and sway control, additional structure of a type disclosed in U.S. Pat. No. 5,456,525 is required to provide pressurized fluid to accomplish the desired function.
A primary object of the present invention is to provide a brake system with a master cylinder having a first mode of operation that responds to an operator input to develop pressurized fluid and effect a corresponding first brake application and having a second mode of operation that responds to an electronic input to develop pressurized fluid and effect a corresponding second brake application.
According to this invention, a brake system is equipped with a master cylinder having a housing with a bore therein. The bore is connected to a reservoir by a radial port and an axial port and to the brake system through first and second outlet ports. First and second pistons are positioned in the bore by a first resilient member located between the first and second pistons to define limits for a first chamber and by a second resilient member located between the second piston and the bottom of the bore to define limits for a second chamber. The first and second resilient member each include a first spring and a second spring. The first spring is caged between a first retainer and a second retainer by a linkage member. The linkage member has a stem with a head that engages the first retainer and an end that engages the second retainer to hold the first spring between the first and second retainers. The second spring is located between the head and the first retainer member. The first chamber is connected to the radial port through an axial passage in the second piston while the second chamber is connected to the axial port in the housing. The first and second pistons respond to an input force applied to the first piston by a first input member by initially moving within the bore to compress the first springs of the first and second resilient members and thereafter allow the second springs to simultaneously move the head on the stem of the linkage member of the first resilient member into engagement with a seat on the second piston to seal the axial passage and the head on the stem the linkage member of the second resilient means into engagement with a seat for the axial port of the housing to terminate communication between the bore and the reservoir. Further movement of the first and second pistons by the input force from the operator into the first and second chambers develops pressurize fluid which is supplied to the brake system through the first and second outlet ports to effect a first brake application. A second input member has a plunger sealingly located in the axial port of the housing. The plunger has a cylindrical base that forms a surface for a seat for the head on the head of the linkage member of the second resilient member. The plunger has a passage or concentric axial bore through which the second chamber is connected to the reservoir. A solenoid associated with the plunger is connected to an electronic control unit (ECU). The ECU receives inputs relating to the operational conditions of the vehicle and environment and if the ECU determines that the vehicle is experiencing or may experience undesirable conditions that may effect the safe operation of the vehicle, an input signal is supplied to activate the solenoid valve. Activation of the solenoid valve causes the plunger to initially move its cylindrical base into the second chamber and compress the first spring and allow the second spring to urge the head on the stem into engagement with the seat formed on the plunger to seal the second chamber from the reservoir. Thereafter, further movement of the plunger into the second chamber pressurizes fluid in the second chamber that is communicated to the rear wheel brake to effect a second brake application and attenuate the effect of the undesirable conditions.
An advantage of this brake system is provided by a master cylinder with first and second input members to develop pressurized fluid to effect corresponding first and second brake applications.
A still further advantage of this invention is provided by a second input member which moves a seat to initially seal an operational chamber from a reservoir and thereafter move to develop pressurized fluid in the operational chamber to effect a brake application.
Another advantage this invention resides in a solenoid valve that has a plunger with a seat thereon through which an operational chamber is communicated to a reservoir that is activated to engage a poppet on a compensation valve and moves into the operational chamber to develop pressurized fluid therein and effect a brake application.
A still further advantage of the present invention resides in an ability to simultaneous actuate of first and second input member of a master cylinder to reduce the actuation time in the development of pressurized fluid to effect a brake application.